The main objective of this research is to establish a genetic basis of male recombination-mutator system (Mr-system) of Drosophila melanogaster, and to understand its significance in evolution. Following specific points will be studied intensively. 1. One of the genetic characteristics of Mr elements is to cause distorted transmission frequency: i.e., from Mr heterozygous male parent, Mr segregates to the progeny in frequency (equals K) less than Mendelian expectation of 0.5. Mr also seems to cause reduction in other zygotic fitness components such as viability and fertility. How such a "bad" element can exist in natural populations in high frequency? One possibility is that, although K is smaller than 0.5 in laboratory genetic background, it may be larger than 0.5 in natural genetic background. A preliminary results suggested it is the case. This will be confirmed in a large-scaled experiment. 2. Segregation Distorter (SD) in D. melanogaster is, like Mr, a system which shows distorted transmission frequency. In this system the k value can be larger than, equal to, or smaller than 0.5 depending upon genotypes in the elements of this system. Recent new model of segregation distortion proposed suggested that the Mr system may well be included in the same model. This will be examined by combining Mr element with various components of SD system. 3. It is now well established that natural populations of D. melanogaster in South Texas contain suppressor X chromosome in high frequency, which suppress distorted transmission frequency under laboratory genetic background. This situation will be examined under natural genetic background. 4. Distorted transmission frequency is due to sperm dysfunction. Electron microscopic observations will be continued to examine more precisely how and when in spermiogenesis the dysfunction initiates and develops, and how and when suppressor X chromosome suppress dysfunctional process.